This is to avoid any confusion caused by g_SYNC_EDGE and g_EDGE
generics used in gc_sync, gc_sync_ffs and gc_sync_edge modules.

Also use capitals for generics as defined by our coding style.

Signed-off-by: Dimitris Lampridis <dimitris.lampridis@cern.ch>

Signed-off-by: Dimitris Lampridis <dimitris.lampridis@cern.ch>

This is especially beneficial when trying to meet timing in the GN4124 core (on Spartan6), where the
async FIFOs are clocked on one side at 200MHz. Apparently, the KEEP_HIERARCHY attribute makes it
much easier for ISE 14.7 to reach timing closure.

It also helps in general to ensure that the synchronisation structures remain intact and do not get
merged in unpredictable ways with other parts of the design.

Signed-off-by: Dimitris Lampridis <dimitris.lampridis@cern.ch>

hdl: add RLOC constraint to gc_sync_ffs in order to keep the two FFs as close as possible to each other